Titanium alloy sheet for electrode

ABSTRACT

Disclosed is a titanium alloy sheet for electrode, including at least one of 0.1 to 1.0% by mass of Al and 0.1 to 1.0% by mass of Si, with the balance being Ti and inevitable impurities, wherein the total content of Al and Si is 0.2 to 1.0% by mass and an average grain size is 5 to 20 μm.

TECHNICAL FIELD

The present disclosure relates to a titanium alloy sheet for electrode,that is used for an electrode of an electrolytic cell in electrolysissuch as soda electrolysis, water electrolysis, or industrialelectrolysis accompanied by generation of oxygen, chlorine or the like.

BACKGROUND ART

In various electrolytic processes including soda electrolysis forproducing sodium hydroxide, chlorine gas and hydrogen gas byelectrolysis of an aqueous sodium chloride solution, an anode usingtitanium as a base material has been widely used. Specifically, therehas been used an electrode material for anode in which a base materialmade of pure titanium (titanium sheet) is processed into a shape havingmany holes, such as an expanded metal or a punched perforated sheet, andthen an electrode catalyst layer containing an electrode catalystcomponent composed of platinum group metal and an oxide thereof isformed on a surface thereof.

When pure titanium is used as the base material, an oxide film existingbetween a surface of pure titanium and the electrode catalyst layer actsas a resistance, thus degrading electrolysis efficiency. If thiselectric resistance can be reduced, the electrolysis efficiency can beimproved, thus enabling reduction in electricity consumption andreduction in costs.

Patent Document 1 discloses an anode capable of improving propertiessuch as energy consumption by using, as a base material, a titaniumalloy containing at least one element selected from the first groupconsisting of aluminum, niobium, chromium, manganese, molybdenum,ruthenium, tin, tantalum, vanadium and zirconium and at least oneelement selected from the second group consisting of nickel, cobalt,iron and copper, and palladium.

Patent Document 2 discloses a method for producing an electrode forelectrolysis in which performances of the electrode are not degradedeven if the amount of an electrode catalyst component used is decreasedby using a base material containing at least one metal selected fromtitanium, tantalum, niobium, zirconium, hafnium and nickel or an alloythereof, and an electrode catalyst component with a predeterminedcomposition, and applying the electrode catalyst component underpredetermined conditions.

CONVENTIONAL ART DOCUMENT Patent Document

-   Patent Document 1: JP 5616633 B1-   Patent Document 2: JP 5548296 B1

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, even if the electrodes (anodes) mentioned in Patent Documents 1and 2 are used, the electrolytic efficiency may not be sufficient.

The embodiment of the present invention has been made in view of theforegoing circumstances, and it is an object thereof to provide a sheetmaterial for electrode, that can be used as a base material forelectrode and enables reduction in electric resistance when an electrodecatalyst layer is formed on a surface, thus enabling realization of highelectrolysis efficiency of an electrode using this base material.

Means for Solving the Problems

The titanium alloy sheet for electrode according to the embodiment ofthe present invention can be used as a base material for electrode. Thetitanium alloy sheet for electrode is a titanium alloy sheet includingat least one of 0.1 to 1.0% by mass of Al and 0.1 to 1.0% by mass of Si,the total content of Al and Si being 0.2 to 1.0% by mass, wherein thebalance is composed of Ti and inevitable impurities and an average grainsize is 5 to 20 μm.

It is preferred that the titanium alloy substrate for electrodeaccording to the embodiment of the present invention has an oxide filmcontaining at least one of Al and Si on a surface thereof, and the totalcontent of Al and Si in the oxide film is 0.08 to 0.55% by mass.

Effects of the Invention

The titanium alloy sheet for electrode according to the embodiment ofthe present invention can be used as a base material for electrode andenables reduction in electric resistance when an electrode catalystlayer is formed on a surface thereof. Therefore, when using for anelectrode, high electrolysis efficiency can be obtained.

MODE FOR CARRYING OUT THE INVENTION

Since titanium is metal having extremely high activity, even if an oxidefilm existing on a surface of pure titanium or a titanium alloy sheet isremoved, new oxide film is immediately formed. Therefore, when puretitanium or a titanium alloy is used as a base material and an electrodeis produced by providing an electrode catalyst layer on a surfacethereof, it is difficult to avoid that an oxide film is interposedbetween the metal portion of the pure titanium or titanium alloy of thebase material and a contact layer.

In view of such circumstances, the inventors of the present inventionhave intensively studied a method of reducing electric resistance (e.g.,contact resistance) between a base material and an electrode catalystlayer when the electrode catalyst layer is formed on a surface of thebase material, on the premise that an oxide film exists on a surface ofthe base material.

As a result, as will be mentioned in detail below, it has been foundthat when using, as a base material, a titanium alloy sheet including atleast one of 0.1 to 1.0% by mass of Al and 0.1 to 1.0% by mass of Si,the total of the Al content and the Si content being 0.2 to 1.0% by massin which an average grain size is 5 to 20 μm, and an electrode catalystlayer is formed thereon, the electrical resistance can be reduced, thuscompleting the embodiment of the present invention.

By controlling the composition and the average grain size in this way, acertain amount of at least one of Al and Si exists in an oxide filmformed on a surface, thus making it possible to suppress the growth ofthe oxide film and improving the adhesion between the oxide film and theelectrode catalyst layer, leading to reduction in electric resistance.

Therefore, the total content of Al and Si in the oxide film ispreferably 0.08 to 0.55% by mass.

The titanium alloy sheet for electrode according to the embodiment ofthe present invention will be described in detail below.

As mentioned above, an oxide film is inevitably formed on the surface ofthe titanium alloy. Accordingly, the term “titanium alloy sheet” as usedherein is a concept including the embodiment in which an oxide film isformed on the surface. In principle, the composition mentioned below isthe composition of the metal portion excluding the oxide film on thesurface. However, since the oxide film is formed within a short timeeven if it is removed as mentioned above, it is often difficult tocomplete composition analysis in a state where the oxide film isremoved. The thickness of the oxide film formed on the surface is, forexample, about 20 nm or less, and the amount of the oxide film isoverwhelmingly smaller than that of the metal portion. Therefore, theresults of composition analysis performed using a bulk sample with anoxide film formed thereon may be regarded as the composition of atitanium alloy sheet. For example, it is possible to use a method thatis generally used for composition analysis such as ICP emissionspectroscopy.

When the composition of raw materials used for blending is clear, valuescalculated from the composition of raw materials and amounts may beused.

1. Composition

In order to contain at least one of Al and Si in an oxide film, thetitanium alloy sheet for electrode according to the embodiment of thepresent invention includes at least one of 0.1 to 1.0% by mass of Al and0.1 to 1.0% by mass of Si. The total content of Al and Si is 0.2 to 1.0%by mass. In the case of including only Al, the content of Al becomes0.2% by mass or more in order to satisfy 0.2% by mass that is the lowerlimit of the total content of Al and Si. In the case of including onlySi, the Si content becomes 0.2% by mass or more in order to satisfy 0.2%by mass that is the lower limit of the total content of Al and Si. Thebalance is composed of Ti and inevitable impurities.

If the Al content is less than 0.1% by mass, sufficient Al does notexist in the oxide film, and it is impossible to sufficiently obtain theeffect of suppressing the growth of the oxide film and improving theadhesion to the electrode catalyst layer by Al. If the Si content isless than 0.1% by mass, sufficient Si does not exist in the oxide film,and it is impossible to sufficiently obtain the effect of suppressingthe growth of the oxide film and improving the adhesion to the electrodecatalyst layer by Si.

In order to sufficiently obtain the effect of suppressing the growth ofthe oxide film and improving the adhesiveness with the electrodecatalyst layer, it is possible to sufficiently include at least one ofAl and Si in the oxide film by containing Al and Si in the total contentof 0.2% by mass or more. This makes it possible to reduce the electricresistance and to improve the electrolysis efficiency.

It is possible to exemplify, as the electrode catalyst layer, a layermade of a platinum group metal and/or an oxide thereof.

Meanwhile, if the Al content exceeds 1.0% by mass or the Si contentexceeds 1.0% by mass, or the total of the Si content and the Al contentexceeds 1.0% by mass, the hardness increases, thus degrading theprocessability. The titanium alloy sheet for electrode is usually usedafter being processed into a shape having many holes, such as anexpanded metal or a punched perforated sheet. However, it becomesdifficult to process into such a shape.

Preferably, the titanium alloy sheet for electrode includes at least oneof 0.3 to 0.5% by mass of Al and 0.3 to 0.5% by mass of Si, and thetotal content of Al an Si is 0.6 to 0.9% by mass.

2. Grain Size

The titanium alloy substrate for electrode according to the embodimentof the present invention has an average grain size of 5 μm or more and20 μm or less.

By setting the average grain size at 20 μm or less, it is possible toimprove the adhesion between the oxide film on the surface and theelectrode catalyst layer. One reason is that the surface roughness tendsto decrease if the average grain size is 20 μm or less. In addition tothis, the other reason is that, by setting the average grain size at 20μm or less, at least one of Al and Si can be contained in the oxide filmin the greater amount even with the same composition.

Si and Al tend to be easily concentrated in grain boundaries. When anoxide film is formed, Si and Al in grains do not enter the oxide filmbut to be expelled to the metal portion. Meanwhile, Si and Al in thegrain boundary tend to be incorporated into the oxide film. Therefore,by decreasing the average grain size thereby increasing the grainboundary and concentrating a greater amount of Si and Al in the grainboundary, a sufficient amount of Si and/or Al can be contained in theoxide film, thus enabling an improvement in adhesion between the oxidefilm and the electrode catalyst layer. Containing a sufficient amount ofSi and/or Al in the oxide film also has the effect of suppressing thegrowth of the oxide film. This makes it possible to reduce the electricresistance and to improve the electrolysis efficiency.

If the average grain size exceeds 20 μm, it is impossible tosufficiently obtain the above-mentioned effect of improving theadhesion. Meanwhile, if the average grain size is less than 5 μm, thehardness increases, thus degrading the processability. The titaniumalloy sheet for electrode is usually used after being processed into ashape having many holes, such as an expanded metal or a punchedperforated sheet. However, it becomes difficult to process into such ashape.

The average grain size is preferably 10 μm or more and 15 μm or less.

The average grain size can be determined by a section method using theresults of structure observation with an optical microscope.

3. Al Content and Si Content in Oxide Film

By setting at the above-mentioned composition and average grain size, asufficient amount of at least one of Al and Si can be contained in theoxide film. This makes it possible to improve the adhesion between theoxide film and the electrode catalyst layer. As a result, the electricresistance between the base material and the electrode catalyst layercan be reduced, thus enabling an improvement in electrolysis efficiency.

The oxide film formed on the surface of the titanium alloy sheetaccording to the embodiment of the present invention preferably containsat least one of Al and Si, and the total content of Al and Si is 0.08 to0.55% by mass.

When the oxide film the oxide film contains Al and does not contain Si,the content of Al in the oxide film is preferably 0.08 to 0.55% by mass.When the oxide film contains Si and does not contain Al, the content ofSi in the oxide film is preferably 0.08 to 0.55% by mass. When the oxidefilm contains Al and Si, the total content of Al and Si in the oxidefilm is preferably 0.08 to 0.55% by mass. This makes it possible to moresurely obtain the effect of suppressing the growth of the oxide film andthe effect of improving the adhesion between the oxide film and theelectrode catalyst layer. As a result, it is possible to more surelyreduce the electrical resistance between the base material and theelectrode catalyst layer, thus enabling an improvement in electrolysisefficiency.

If the total content of Al and Si is more than 0.55% by mass, thehardness of the oxide film may increase and the abrasion of the tool orthe like may be accelerated in the case of processing into a shapehaving many holes, such as an expanded metal or a punched perforatedsheet. Therefore, the total content of Al and Si is preferably 0.55% bymass or less.

More preferably, the oxide film contains at least one of Al and Si, andthe total content of Al and Si is 0.10 to 0.40% by mass.

The Al content and the Si content in the oxide film can be measured byperforming composition analysis using EDS attached to TEM during TEMobservation.

4. Method for Producing Titanium Alloy Sheet for Electrode

The method for producing a titanium alloy sheet for electrode accordingto the embodiment of the present invention will be described below.

A cast billet such as bloom or slab with a desired composition isobtained by melting and forging as needed. It is possible to use amethod that is commonly used for melting a titanium alloy, such as VAR(vacuum arc remelting). A small amount of a sample may be obtained bybutton arc melting or the like.

The thus obtained cast billet such as bloom or slab is heated to 750° C.to 850° C. and then subjected to hot-rolling to obtain a hot-rolledsheet. Heating may be performed in the atmosphere, for example, by openflame of burners disposed in upper and lower portions of a heatingfurnace. As an example of the finish thickness of hot-rolling, 3 mm to 5mm can be exemplified.

Subsequently, annealing is performed to remove processing strain. In thesheet material after annealing, oxidation scale and an oxygen diffusionlayer exist on the surface due to heating of hot-rolling and annealing.If the oxidation scale and the oxygen diffusion layer remain, theelectric resistance increases, thus degrading the electrolysisefficiency when used as an electrode. During cold-rolling, they cancause flaws. Therefore, there is a need to remove the oxide scale andoxygen diffusion layer. For example, they can be removed by pickling.

The thickness (total thickness) L (m) of the oxidation scale and oxygendiffusion layer depends on the heating temperature T (K) and the heatingtime t (second) and can be determined by the following equation (1):

L=2(Dt)^(0.5)  (1)

where D=D₀×EXP(−Q/(RT)), diffusion coefficient D₀=5.08×10⁻⁷ m²/second,activation energy Q=140 kJ/mol, and gas constant R=8.3144.

Therefore, when the oxidation scale and the oxygen diffusion layer areremoved by pickling or the like, the removal amount (pickling amount)are required to exceed L. Pickling can be performed using fluonitricacid or the like.

After removing the amount exceeding L from the surface by pickling orthe like, the hot-rolled sheet is rolled to a predetermined thickness bya cold-rolling step.

After the cold-rolling, in order to remove processing strain similar tothe case after the hot-rolling step, the cold-rolled sheet is placed ina furnace to conduct an annealing treatment in the atmosphere. Bysetting the heating temperature at this time to 780 to 830° C., it ispossible to control the average grain size within a predetermined range.

Since the oxidation scale and oxygen diffusion layer exist on thesurface of the sheet material even after the cold-rolling and subsequentannealing, the thickness L of the oxidation scale and oxygen diffusionlayer is determined by the equation (1), and the surface is removed bypickling or the like by the amount exceeding L thus obtained. Picklingcan be performed using fluonitric acid or the like.

Since titanium is metal that is active with oxygen, an oxide film isformed on a surface of a titanium alloy sheet immediately afterpickling. During the formation of the oxide film, Al and Si existingnear the surface are incorporated into the oxide film. When the oxygendiffusion layer exists near the surface because of insufficientpickling, Al and Si are hardly incorporated into the oxide film due tointerference of oxygen, thus failing to contain a sufficient amount ofAl and/or Si in the oxide film.

Therefore, there is a need to surely remove the surface by the amount(thickness) of L or more determined by the equation (1) by pickling orthe like.

Thus, the titanium alloy sheet for electrode according to the embodimentof the present invention can be obtained.

Examples 1. Fabrication of Test Materials

The present invention will be described in more detail by way of thefollowing Examples. It should be noted that the following Embodimentsare intended to facilitate understanding of the present invention and donot limit the scope of the present invention.

Test materials were fabricated in the following way.

Ingot of titanium alloys with each composition shown in Table and havinga size of about 40 mm in diameter×20 mm in height was manufactured usingbutton arc melting.

The ingot was heated to 1,000° C. and subjected to forging to fabricatethe forged material with a size of 10 mm in thickness×35 mm in width×75mm in length. After surface grinding and heating at 850° C. for 120minutes, hot-rolling was performed to obtain a sheet with a size of 3.5mm in thickness×35 mm in width×165 mm in length. Thereafter, annealingat 750° C. for 20 minutes was performed in the atmosphere.

Then, the sheet thus obtained was pickled with fluonitric acid. Eachthickness L of an oxidation scale and an oxygen diffusion layerdetermined by the equation (1) was about 80 μm. In order to surelyremove the oxide scale and oxygen diffusion layer, the removal amount(pickling amount) by pickling was set at 120 μm on one side (240 μm onboth sides).

Subsequently, cold-rolling was performed at room temperature to obtain asheet with a size of 0.52 mm in thickness, 36 mm in width×1,000 mm inlength.

Then, this sheet was annealed in the atmosphere at 800° C. for 2minutes.

Then, the sheet was pickled with fluonitric acid. Each thickness L of anoxidation scale and an oxygen diffusion layer determined by the equation(1) was about 6 μm. In order to surely remove the oxide scale and theoxygen diffusion layer, the removal amount (pickling amount) by picklingwas set at 10 μm on one side (20 μm on both sides).

2. Evaluation Results of Test Materials

The test material thus obtained was cut into a predetermined size andcross-sectional observation (at a magnification of 100,000 times) wasperformed using a transmission electron microscope (TEM). Using the thusobtained micrograph (TEM image), after selecting five positions wherethe thickness of the oxide film is considered to be representative, thethickness of the oxide film in this point was measured and the averagethereof was taken as the thickness of the oxide film. The results areshown in Table 1.

Quantitative analysis with EDS was also performed and the componentvalues near the center in the thickness direction of the oxide film weremeasured at five positions randomly selected, and then each content ofAl and Si in the oxide film was determined from the average. The resultsare shown in Table 1.

The average grain size was measured at one field of view having an areaof 520 μm×860 μm by a section method using the results of structureobservation with an optical microscope (at a magnification of 100times). The results are shown in Table 1.

Vickers hardness (load of 10 kgf) was measured at five positions nearthe center in the thickness direction of the cross section and theaverage was taken as hardness. The results are shown in Table 1.

TABLE 1 Alloy composition Thickness of Component in oxide film AverageContact (% by mass) oxide film (% by mass) grain size Hardnessresistance Al Si Ti (nm) Al Si (μm) Hv (mΩ · cm²) Example 1 0.10 0.00Bal. 7.32 0.08 0.00 18.6 147 4.5 Example 2 0.30 0.00 Bal. 7.15 0.18 0.0015.4 150 5.5 Example 3 0.50 0.00 Bal. 6.80 0.25 0.00 11.6 183 4.0Example 4 1.00 0.00 Bal. 6.78 0.28 0.00 9.6 199 3.1 Example 5 0.00 0.50Bal. 6.73 0.00 0.50 5.1 199 4.1 Example 6 0.00 1.00 Bal. 6.69 0.00 0.535.0 199 3.6 Example 7 0.50 0.35 Bal. 6.80 0.24 0.25 7.9 192 4.0Comparative 0.00 0.00 Bal. 7.71 0.00 0.00 36.2 144 6.5 Example 1Comparative 1.50 0.00 Bal. 6.77 0.35 0.00 10.5 225 — Example 2Comparative 0.00 1.50 Bal. 6.65 0.00 0.58 4.3 214 — Example 3

As is apparent from Table 1, the test materials of Examples 1 to 7 andComparative Example 1 exhibit hardness (Hv) of less than 200 and haveexcellent processability. Meanwhile, the test material with excess Sicontent of Comparative Example 2 and the test material with excess Alcontent of Comparative Example 3 exhibit hardness of 200 or more andhave insufficient processability.

3. Measurement of Contact Resistance

With respect to the test materials in which processability was ratedgood because of having hardness of 200 or less of Examples 1 to 7 andComparative Example 1, an electrode catalyst layer was formed on asurface and the contact resistance was measured.

After shot blasting and pickling, the above-mentioned test material wascut into a size of 20 mm in width×40 mm in length and an electrodecatalyst layer was formed on both sides. Specifically, a catalystlayer-forming solution prepared by mixing a ruthenium chloride acidsolution, an iridium chloride acid solution and titanium chloride wasapplied to a surface of each sample after subjecting to shot blastingand pickling, placed in a dryer (inside temperature: 75° C.) and driedfor 2 minutes. The dried sample was placed in an atmosphere heattreatment furnace set at a furnace temperature of 475° C., held for 10minutes and then taken out. Lamination was performed by repeating theoperation from application of the catalyst layer-forming solution toheat treatment (holding) five times. Finally, a heat treatment wasperformed at 500° C. for 60 minutes to form an electrode catalyst layer.

The contact resistance of the sample on which the electrode catalystlayer was formed was measured.

The sample after formation of the catalyst layer was interposed betweengold sheets, and two gold sheets between which the sample was interposedwas further interposed between two copper electrodes under a load of 10kgf so that the contact area became 1 cm². In this state, a current wasapplied between two copper electrodes and the voltage at that time wasmeasured by a voltmeter disposed between two gold sheets. The contactresistance was determined from the current applied and the measuredvoltage.

The results are shown in Table 1. All the samples of Examples 1 to 7exhibit low contact resistance of 3.1 to 5.5 mΩ·cm² and can realize highelectrolysis efficiency. Meanwhile, the sample with insufficient Si andAl contents and excess average grain size of Comparative Example 1exhibits large contact resistance of contact resistance of 6.5 mΩ·cm².

This application claims priority based on Japanese Patent Application2016-163915 filed on Aug. 24, 2016, the disclosure of which isincorporated by reference herein.

1. A titanium alloy sheet for electrode, comprising at least one of 0.1to 1.0% by mass of Al and 0.1 to 1.0% by mass of Si, with the balancebeing Ti and inevitable impurities, wherein: a total content of Al andSi is 0.2 to 1.0% by mass; and an average grain size of the titaniumalloy sheet is 5 to 20 μm.
 2. The titanium alloy sheet for electrodeaccording to claim 1, comprising an oxide film containing at least oneof Al and Si on a surface thereof, wherein a total content of Al and Siin the oxide film is 0.08 to 0.55% by mass.